Air lock method of increasing tunnel capacities



C. A. LEE, JR

Filed Feb. 20,

Oct. 27, 1931.

AIR LOCK METHOD oF INCREASING TUNNEL CAPACITIES Patented Oct. 27, 1931 UNITED STATES rPATENT OFFICE CHARLES A. LEE, JR., OF WESTIORT, CONNECTICUT, ASSIGNOR TO DOHERTY RESEARCH COMPANY, F YORK, N. Y., A CORPORATION OF DELAWARE AIR LOCK METHOD OF INCREASING TUNNEL CAPACITEES Application led February 20, 1930. Serial No. 430,092

This invention relates to tunnels or conduits for transporting liquid, and more particularly to improvements in method and means for increasing the liquid iiow capacities of tunnels and other conduits.

Tunnels and other conduits designed for transporting liquids under flow line lconditions are usually constructed on moderately flat slopes or grades. It has been observed that when the hydrostatic head on such conduits is increased withi'the object of increasing the liquid carrying capacity of the conduit by causing the conduit to tlow full, such increase in head does not cause a correspondingly proportional increase in flow, because the rictional resistance to low is increased out of all proportion to the head increase as soon as the conduit is filled and the liquid comes in contact with the roof.

The primary object of the present invention is to provide method and means for increasing the liquid carrying capacity of tunnels and conduits used in transporting liquids. Another and more specific object of the invention is to provide method and means whereby the upper surface of liquid which is being transported through a tunnel under a head sufficient to cause the tunnel to run full, ycan be kept away from the roof'of the tunnel without at the same timevmaterially decreasing the cross-sectional area of the liquid in the tunnel, thereby increasing the hydraulic radius and decreasing the frictional flow resistance of the tunnel.

In order to accomplish these objects the invention contemplates providing and maintaining a series of shallow air pockets between the roof of the conduit and the liquid at a pressure sufficient to hold the liquid away from the roof.

With the above and other objects and features in view, the invention consists in the method and means for increasing the iiow Vcapacity of tunnels and conduits for carrying liquid, which is hereinafter described and particularly dened in the claims.

The various features of the invention are illustrated in the accompanying drawings,

showing a conduit of square cross-section equipped with means for maintaining a series of shallow air pockets between its roof and the liquid flowing therethrough; and

Fig. 2 is a vertical cross-sectional end view, taken on the line 2--2 of Fig. 1.

In the drawings a tunnel l0 of substantially square cross-section is shown as hav- -ing its floor `and side walls to a height 12 lined with a substantailly smooth surface of concrete or the like, and its roof unlined. A conduit of square cross-section has been shown for the purpose of illustrating the invention because application of the present invention to a conduit of square cross-section gives a greater increase in liquid flow capacity than would be the case with a conduit of round or egg-shaped section; though the increase'in capacity of a square tunnel is of course less than that which would be effected by application of the invention to a tunnel of .flat top basket or rectangular section, having greater :width than depth.

Let it be assumed, for example, that the tunnel l0 is 5000 ft. long and is constructed on 4a slope of 0.5 it. per 1000 tt. orn length;` Let '.015, which is a conservative value for ordinary concrete tunnel lining, then applying Cnezys formula we have the following conditions: t .Y Av (area) 100 disq. ft. l P (wetted perimeter) :30 ft.

R 333 a. Y I V (velocity) =`5.0 ft. per second.

Q, (capacity) :500 cu. it. per second.

If now the head Aof'water at'the tunnel entrance is increased so that the tunnel will iiow full to the roof, and if it be assumed sumed that Vthe tunnel is completely lined with concrete (including the roof) l'and that the total effective head from intake to outlet is ft., then (applying Cnezys formula) If it.ivere.possible:to hold the Water from the roof by means of a continuous-film of air Without reducing the cross-sectional'area of thetunnel, then (applying Cnezys formula) R1: 3.33V ft.

,.-V .Q41 990 cu. ft. per second.

vfiowingf therethrough. According :toA thev i v:v 9.9 fr.' 2

Obviously it is not possible'or. Apracticable maintainsuch a thin continuous air film lbetween the roof of the' tunnel-and the-liquid present invention, the-flow capacity of a tunnel of the foregoing` dimensions is increased vby forming and maintaining a series of sha1` low `air pockets under the roof of the tunnel v.throughout its length, so as to holdl the Water away from the roof. In order to form such Vair pockets a number-of smooth faced concrete roof contractions or air dams 14, of a somewhat'fiat V shaped cross-section, are

V.built at widely spaced intervals along the `roof of the tunnel and extending fromside. iIto side thereof (Fig. 2). These roof contractions 14 serve to decrease the effective crosst sectional area of the tunnel in the vertical `planes of their apexes andl thereby format such points in .the tunnel the equivalents of V-what might be termed semi-Venturi throats.

In a tunnel of the slope and dimensions'of the examples previously given, the contractions 14 may, for example, be spaced 1000 ft. apart and .at these Vpoints the tunnel may, for example,`be decreased in heightfrom lOl-ft. to 81/2'ft.Y

Air isintroduced into the-tunnela short distance ,below the intake 16 through "av pipe Y 18. The outlet of pipe V18 is located'behind an air dam 19. at ahigher levelthan the'apex of dam`19 so that air will not'escape through -the tunnel intake. The air` isrintroduced at a flow offered-at points Where 'the water surface -130 available toovercome friction in the Ytunnel is, it Will be assumed, 10feet. 1 Since lthe tun` lnel is divided by the roof contractions (14) intofive sections each 1000 ft.long, thenY one- Vfifth of'the'eflective head, or 2`feet', can be utilized in each section for overcoming resistance to flow. The .pressure of air in the rst shallow pocket or lock 22 thus formed is of courseuniformover the entire surface area Y Y of Water beneath such pocket, and is sufiicient -toi holdthe Water a-Wayfrom the roof. The velocity imparted tothe -vvater by the-velocityheadat the entranceofpthetunnel Will not only be maintained but increased as the liquid flows downwardlythrough the section ofthe-tunnel under airpocket. 22, by the sur- `face slope. Which the Water .Will automatically assume infiowing from. the'V entrance of the tunnel tothe Venturithroat 23 formed by the first .main air-:dam 'orf roof contraction 14. .Since Q. (i. eithevol'ume fiovv) is the "same at all Vpointszin the .tunneland since .the Vcross-sectional .area of thetunnel is greateratthe upper end ofeach 1000 feet section (100 sq. ft.)-= than at the loWer. end (85 sq. ft.), .1

the slope ofthe upper surfaceof the liquid gradually increases. to maintainthe increase in velocity flow, the total surface drop in such 1000 it. section being 2fft.. `When the air pressure .in the first pocket 22 .exceeds the 10'ftuofeffective Waterhead, the Water surface will beislightly'depressedlat the firstroof `eontraction 14 by air flowing with the Water Yfrom-the first section-or airpocket into aseczond air pocket 24 in the second 1000 ft. section of the tunnel. The airpressurein the second .pocket 24 Will be maintained at tvvo ft. less head than' that in thevfirst pocket.V Assum- -ing acontinuous supply ofair through the pipelS, theair pressures in the successive lair `pockets Will .be automatically regulated and hWill decrease-by decrements of 2 ft. of head `from a-pressure of 10 ft. inthe first pocket .to zero at the tunnel outlet. Since .the effective fiovvr-areas, pressure .difference and slope are -the samebeneathjall'pocketsa uniform flow uof liquid -vvill .be Vmaintained throughout .the ivholestunnel f y l j iUnder the .conditions lassumed in the above example, the calculatedfiow capacity` of the 'tunnel with the air pockets dimensioned as above-#described .is 910;cufft.' per second. 'f lhislfflovv-4 capacity is 410 cu. per second greaterthan the capacity of the tunneloper- .ating Von fl'ovvflineV grade( or vkan .increase of 85%, andis 100 cu. ft; per seconder 121/2% 'gr-eater than the capacityofa completely .lined tunnel operating under the same head. While there isof course a slight resistance to comes in contact with the roof contractions 14, the total length of such points of contact 1s very small, particularly when air is fed to the tunnel at a reasonably rapid rate. lf it be assumed that the total length for 'such points of contact is 100 ft. the consequent loss of head is calculated to be only about 0.25 ft. Such additional head loss would decrease the calculated capacity indicated below the figure given, but conservative calculations indicate that by means of five roof contractions 14 of the dimensions illustrated and the air pockets maintained thereby, the capacity of the 5000 ft. tunnel of the example is increased 75% over flow line conditions and at least 10% over the capacity of a completely lined tunnel operating under the same head.

In case the liquid head on the tunnel should be less than the maximum of 10 ft. eifective head assumed in the foregoing example, the pressure at which air is introduced into the tunnel should be reduced proportionately to a value approximately equal to the effective head. In every case the reduction of air pressure in the successive air pockets will be automatic, just as when the maximum head is operative, and a. reduced Vflow will be maintained by the flatter surface slope in the tunnel. As the effective head is reduced the drop of water just below the roof contractions will be less, but the depth directly under the contractions will remain constant until the effective head is reduced to less than that required to maintain free surface flow, when of course the surface slope will be parallel to the slope of the tunnel.

rlhe principles of the present invention may be applied to existing pressure tunnels or to portions of such tunnels, with resulting increase in liow capacity of the tunnels when operating under the same effective head. The quantity of air that is required to maintain the air pockets throughout the length of the tunnel or conduit depends upon the capacity of the water or other liquid carried by the conduit for air absorption, and on the amount of air leakage. Most liquid tunnels through rock can be readily and effectively sealed against air leakage by caulking, plastering, or grouting the cracks and seams in the rock. Normally the volume of air absorbed by the water or other liquid is not a serious item.

F or regulating the rate of admission of air to the tunnel, a valve 26 is mounted in the air supply line 18 and arranged for automatic operation by means of a float 28 located in the forebay 30.

Wherever additional entrance head is available, and where the roof of the tunnel is of such nature thatit can be made practically air tight, flow line tunnel capacity can be readily increased by the present air lock system at a cost well below the cost of completely lining the roof for operation as a pressure tunnel, with the resulting additional advantage of higher flow capacity over that of a fully lined pressure tunnel Without such air locks.

The invention having been thus described, what is claimed as new is:

1. The method of increasing the liquid How capacity of a conduit of moderate slope which comprises, establishing a hydrostatic head on the conduit suiiicient to cause it to run full of liquid, and maintaining a series of shallow air pockets between the roof of the conduit and the surface of the liquid flowing therethrough.

2. rlhe method of increasing the liquid flow capacity of a conduit of moderate grade which comprises, establishing a hydrostatic head on the conduit sutiicient to cause it to run full of liquid, and maintaining a series of shallow air pockets between the roof of the conduit and the surface of the liquid, the air pressure in each of said pockets being approximately equal to the static head on the liquid flowing' beneath said pocket.

3. The method of increasing the liquid flow capacity of a conduit running full of liquid which comprises, fractionally contracting the cross-sectional area of the body of liquid flowing through the conduit at spaced intervals, and maintaining a shallow air pocket between the roof of the conduit and the flowing liquid on the upstream side of each of said contractions.

al. ln a conduit for transportino liquid, a series of smooth faced roof contractions located at spaced intervals throughout the length of the conduit and extending from side to side thereof, and means operable in conjunction with said roof contractions to maintain a shallow air pocket on the upstream side of each of said roof contractions between the conduit roof and the surface of liquid flowing therethrough.

5. In a conduit for transporting liquid having a forebay at its inlet end, a plurality of roof contractions located at spaced intervals longitudinally of the conduitv whereby the eifect've cross-sectional area of the tunnel is fractionally reduced, and means for autoatically maintaining shallow air pockets under the roof ofthe tunnel in front of each of said roof contractions during periods in which the head of liquid in said forebay is sufficient to cause the conduit to run full.l

ln testimony whereof I aiiix my signature.

CHARLES A. LEE, JR. 

